Sulfur Cycle Lecture Outline

1. Review of Important Vocabulary
   1. Assimilative vs. Dissimilative sulfate reduction
   2. Oxidation and reduction reactions - Redox
2. Oxidation States of Sulfur
   1. 3 significant forms of sulfur having to do with the Sulfur Cycle
   2. environmental conditions for some sulfur sources
3. Role of microbes in the Sulfur Cycle
   1. Sulfur oxidizing Bacteria (and some Archea)
   2. Sulfur reducing Bacteria

Assimilative Sulfate Reduction -

When inorganic sulfur compounds are reduced by organisms as a source of nutrition they are said to assimilated in a process known as assimilative metabolism.

Normally this assimilative metabolism produces a -SH (sulfhydryl group). The reduced sulfur is converted into cell material in the form of macromolecules such as sulfur containing proteins.

Bacteria, Archea, fungi, algae and higher plants have the ability to use the assimilative metabolism.

Dissimilative Sulfate Reduction -

When the sulfur source is used as an electron acceptor for an energy metabolism it is referred to as a dissimilative metabolism.

In this metabolism large amounts of sulfur are reduced and excreted to the environment.

Environmental conditions for some sulfur sources

Oxidation and reduction reactions - Redox

Redox reactions can be confusing and frustrating, even for people that are involved with them regularly. We are going to apply the rules of oxidation-reduction to the sulfur cycle while keeping it simple. In other words we'll use the KISS principle- Keep It Simple Stupid.

Redox reactions are important for biological systems for management of energy. Some compounds are in a reduced state which means they have a surplus of electrons that can be released. These released electrons are like packets of energy. As these electrons, or packets of energy, are released another compound is accepting them. An electron acceptor is said to be reduced. The chemical process of oxidation-reduction becomes a cycle that is efficiently carried out by microorganisms.

Oxidation

Sulfide (S2-), the most reduced form of Sulfur, is oxidized to elemental sulfur (S0).This is then oxidized into sulfite (SO3) and further oxidized into sulfate (SO4).

Reduction

SO4 is then reduced back into SO3 and reduced to S0 and finally reduced to sulfide.

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Biogeochemcial Cycles

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